A pneumatic sheet feeder is described in U.S. Pat. No. 4,579,330 wherein the upper surface of the carrier is flat and two parallel lateral abutments are provided thereon which have a number of blowing apertures on the facing sides. By blowing air through these apertures towards the sides of the stack of sheets, thin air layers are formed between the bottom sheets of the stack thereby separating them. The bottom-most sheet is then drawn towards a rotatable suction roll provided with holes in which a partial vacuum is created. The bottom-most sheet is discharged from the stack by means of the suction roll.
Considerable leakage occurs in this device when air is supplied to the stack. Primarily the leakage occurs at the front and rear of the stack of sheets, when considered in the direction of sheet transport, and results in a considerable amount of air being required by the device to separate the bottom sheet from the rest of the stack. This, of course, necessitates the use of large fans with their accompanying considerable amount of noise. Moreover, if there are only a small number of sheets in the stack to be discharged, the sheets in the holder will vibrate causing an adverse effect on the sheet separation process and further increasing the noise.
To prevent more than one sheet from being discharged simultaneously in this device, a retaining plate must be used which has to be adjusted to the exact thickness of the sheets in the stack so that only one sheet can pass under the retaining plate and over the rotatable suction roll. In practice it has been found that even a slight deformation of the side edge of the sheet situated directly above the bottom sheet in such a device prevents said sheet from being separated from the bottom sheet. It has also been found that unless air is blown over the entire length of the side edges of the sheets in the stack, the stack bends and deforms in an undesirable way when air is supplied, making separation more difficult. Furthermore, the resistance of the stack to deformation in such cases counteracts the force exerted by the supplied air, so that not only the weight but also the bending resistance of the stack of sheets exerts an influence on the air layer that has formed with the result that the nature of the stack greatly influences the reliability of separation. U.S. Pat. No. 2,812,178 and Dutch Patent Application 295,216 disclose sheet separating devices similar to that described in U.S. Pat. No. 4,579,330.
French Patent No. 1,460,465 discloses a sheet feeder where sheets are drawn from the bottom of a stack without the use of blown air. The sheets are separated by transporting the bottom sheet, using a belt with a suction box inside the circumference of the belt while the other sheets in the stack, which lie directly on top of the bottom sheet, are prevented from being transported simultaneously with the bottom sheet using a preset retaining plate.
U.S. Pat. No. 3,099,442 and French Patent 2.219.665 disclose other devices for separating sheets using blown air. In the device described in U.S. Pat. No. 3,099,442 air is used to draw the bottommost sheet towards the carrier and the stack is lifted using air blown to the side of the flat lying stack. Separation is more difficult owing to the fact that air is not blown over the entire length of the side edges of the stack. This sheet feeder therefore has the drawbacks already stated above for U.S. Pat. No. 4,579,330. The device described in French Patent 2.219.665 uses blown air to discharge a sheet from the top of a stack, not the bottom of a stack.
It would be desirable, therefore, to have a pneumatic sheet separating and discharge device which did not have the above-described disadvantages.